1. Field of the Invention
This invention relates generally to vortex-shedding flowmeters for measuring the amount of fluid passing through a pipe and more particularly, to an apparatus for determining the mass flow rate of the moving fluid.
2. Description of the Prior Art
For many industrial fluid processes such as custody transfer, fuel metering or reactant mixing applications, it is desirable to be able to measure the mass flow rate of the fluid passing through the pipe.
In a conventional vortex-type flowmeter, fluid passing around a shedding body produces a stream of vortices with a generation rate which is proportional to the flow rate (v) of the fluid. A sensor responsive to the vortices produces a signal having a frequency representing the flow rate. Since the cross sectional area of the flowmeter is known, the flow rate signal can then be used for calculating the resulting volumetric flow rate of the fluid in the pipe. If the density (.rho.) of the fluid is also known, the resulting product of volumetric flow rate and density is the mass flow rate. However, it is not possible to provide a simple constant of proportionality to derive mass flow rate from only the direct measurement of volumetric flow rate because density is sensitive to changes in temperature and pressure. For gas applications, this sensitivity is typically so greatly pronounced that a direct measurement of density under operating conditions is necessary in order to obtain reasonably accurate measures of mass flow rates.
Various devices for measuring mass flow have been proposed which do not require fluid density to be measured directly. For example, in U.S. Pat. No. 3,719,073 issued to Mahon, a mass flowmeter is disclosed wherein a vortex-shedding body is used in series with a downstream sensor that detects both the frequency and amplitude of the oscillating flow pattern resulting from the interaction of the moving process fluid and the vortex-shedding body. The Mahon patent teaches that the frequency varies directly with changes of the fluid flow rate (v) and the amplitude varies with changes of a flow characteristic which is the product of the fluid density and the square of the fluid flow rate (i.e., .rho.v.sup.2). As a result, the mass flow rate can be calculated by dividing one sensor signal representing the detected amplitude of the flow pattern by another sensor signal representing the frequency of the flow pattern.
Another prior art mass flowmeter is disclosed in U.S. Pat. No. 3,785,204 issued to Lisi, wherein a vortex-shedding measuring device is combined in series with a differential pressure instrument having two pressure taps. One pressure tap is placed upstream and the other pressure tap is disposed downstream of the vortex-shedding measuring device. Using the measuring device as an obstruction in the moving stream of fluid, a pressure drop is created which is well known to be proportional to the .rho.v.sup.2 flow characteristic. As a result, the calculation of the mass flow rate is similar to that taught in the Mahon patent wherein the measured .rho.v.sup.2 flow characteristic is divided by the fluid flow rate v.
However, a problem inherent in the mass flowmeters described in the Mahon and Lisi patents arises from the axial separation between the device measuring the .rho.v.sup.2 flow characteristic and the device measuring the fluid flow rate. It is known that this separation results in some energy changes primarily in the form of pressure losses and increased turbulence appearing between the two measuring devices. These energy changes limit the accuracy of the mass flow rate measurement since the conditions existing respectively at the two measuring locations are not the same. Thus, in order to increase the measurement accuracy of these prior art flowmeters, particularly for use in low-pressure gas applications, additional devices and circuitry are needed to compensate for errors arising from the aforementioned energy changes. But the use of compensating devices, whether for liquid or gas applications, increases the complexity of both types of prior art flowmeters so that they become more costly and less reliable. Moreover, error compensation becomes more complicated as the axial separation between the two measuring locations is reduced because the correctional factors become more difficult to predict.
Accordingly, there is a need for an improved flowmeter for measuring the mass flow rate of a process fluid without resorting to the use of additional circuitry or devices to compensate for energy changes.